Some of the statements in this presentation constitute "forward looking statements" under the Private Securities Litigation Reform Act of 1995, including, but not limited to, statements on the timing and results of our c

J.P. Morgan Healthcare Conference

Roger J. Pomerantz, M.D. January 11, 2018 President, Chief Executive Officer and Chairman Exhibit 99.1

Some of the statements in this

presentation constitute "forward looking statements" under the Private Securities Litigation Reform Act of 1995, including, but not limited to, statements on the timing and results of our clinical trials, the sufficiency of our financial

resources, and dysbiosis as an underlying cause of disease or failed response to therapy. Such statements are subject to important factors, risks and uncertainties (such as those discussed under the caption "Risk Factors" in the Company's Quarterly

Report on Form 10-Q filed on November 8, 2017 and its other filings with the SEC) that may cause actual results to differ materially from those expressed or implied by such forward looking statements. Any forward looking statements included herein

represent our views as of today only. We may update these statements, but we disclaim any obligation to do so. Forward looking statements

Seres investor highlights Phase 3 stage

company developing microbiome-based therapeutics, a highly promising new area of medicine Leader in microbiome drug development with differentiated capabilities, leading CMC and demonstrated GMP quality, and supportive clinical data Broad pipeline

in infectious and metabolic diseases, inflammation & immunology, including immuno oncology Experienced, highly accomplished leadership team Platform Opportunity Pipeline Team

The microbiome is essential to human

health Infectious Disease A diverse microbiome resists colonization by exogenous pathogens Exposure to broad spectrum antibiotics, and resulting gut microbiome dysbiosis, increase risk for C. difficile infection and colonization / infection by

multi-drug resistant organisms Inflammation and Immunology Microbiome known to alter regulatory T cells and Th17 T cell activation Role in inflammatory bowel disease (Ulcerative Colitis and Crohn's disease) as well as allergy, rheumatoid

arthritis and multiple sclerosis The composition of the microbiome has been demonstrated to impact the efficacy and safety of immuno-oncology checkpoint inhibitors Metabolic Disease Effects on glucose utilization, digestion and bile acid metabolism

Role of microbiome implicated in several metabolic diseases (e.g. diabetes, obesity, liver diseases) Selected references: Infectious disease / C. difficile: Leffler and Lamont, NEJM, 2015; Ulcerative colitis: Paramsothy et al. Lancet, 2017; Moayyedi

et al. Gastroenterology, 2015; Immuno-oncology: Vetizou M et al., Science 2015.; Slvan A. et al., Science 2015.; Dubin et al., Nature, 2016. NASH: Le Roy et al., Hepatology, 2012. Metabolic disease: Perry et al. Nature, 2016, Ridaura VK et al.,

Science 2013; Primary sclerosing cholangitis: Tabibian JH et al., Hepatology, 2016.

Business strategy Prioritize serious

diseases where dysbiosis of the gut microbiome has a causal role Focused R&D Computational biology Basic microbiome research Microbiology Translational science Clinical development Advanced GMP manufacturing World class, differentiated,

microbiome expertise Collaborate with leading academic centers to advance research in promising therapeutic areas Research in new therapeutic areas Inflammatory bowel disease C. difficile infection Immuno-oncology

Research Collaborations Robust

microbiome therapeutics pipeline PRECLINICAL PHASE 1b PHASE 2 Synthetically fermented Infectious Inflammatory SER-301 Inflammatory Bowel Disease (IBD) SER-262 Primary C. difficile SER-401 Immuno-oncology - in combination with anti-PD-(L)1

therapy Biologically sourced SER-287 Ulcerative colitis PHASE 3 SER-109 Recurrent C. difficile SER-155 Prevention of infection and GVHD following hematopoietic stem cell or solid organ transplant Positive Phase 1b results Collaboration

with MD Anderson and Parker Institute Pivotal study

Clostridium difficile Infection

Overview and R&D Programs Leading the Microbiome Revolution

C. difficile infection overview

Infectious disease caused by toxin-producing anaerobic, spore-forming bacteria, resulting in diarrhea, abdominal pain, fever, and nausea Leading cause of hospital-acquired infection in the US Approximately 29,000 deaths/year ~25% of patients with

primary C. diff. recur Risk of relapse increases with each recurrence Multiply recurrent C. difficile infection incidence increased 188% between 2001-2010 Sources: Leffler and Lamont, New England Journal of Medicine, 2015; Ma et al. Annals of

Internal Medicine, 2017.

Microbiome therapeutic intervention

- Race to Repair Hypothetical patient course Healthy person with intact pathogen resistance High diversity microbiome Use of broad spectrum antibiotics Active C. diff. infection Antibiotic treatment of C. diff. infection Potential for

microbiome therapy to reduce risk of C. diff. recurrence Antibiotic mediated C. diff. killing Antibiotic exacerbation of dysbiosis Gastrointestinal microbiome diversity C. diff. bacterial & cytotoxin levels C. diff. recurrence risk C. diff.

outgrowth & cytotoxin production Potential for microbiome therapy to increase bacterial diversity in the GI tract Antibiotic mediated dysbiosis

Phase 3 SER-109 ECOSPOR III study

ongoing FDA Breakthrough and Orphan Drug designation Based on FDA feedback, ECOSPOR III designated as a Phase 3 study Phase 3 study incorporates key learnings from prior clinical efforts: SER-109 dose is approximately 10-fold higher than dose used

in Phase 2 study C. difficile toxin assay to be used at study entry and for primary endpoint Multiply recurrent C. difficile patients, screened by toxin assay All subjects treated with standard of care antibiotics Primary endpoint: C. diff.

recurrence, at up to 8 weeks Safety follow-up to 24 weeks Phase 3 ECOSPOR III (SERES-012) study design SER-109 (n = 160) Placebo (n =160)

SER-262: Synthetic, fermented

Ecobiotic therapeutic candidate for primary C. difficile infection Oral, microbiome therapeutic candidate comprising twelve strains of fermented, rationally-selected bacterial spores Bacterial species selected based on analysis of SER-109

Phase 1b microbiome data, biological and phylogenetic heterogeneity, and preclinical efficacy in C. difficile infection mouse model Data support a mechanism of action in which SER-262 strains compete for C. difficile preferred carbon sources SER-262

strains utilize multiple carbon sources In vitro fermentation

SER-262 Phase 1b dosing study in

patients with primary C. difficile infection Primary Objective Safety and tolerability at 24 weeks Relative risk of C. difficile recurrence compared to placebo at up to 8 weeks Secondary Objectives Microbiome engraftment Time to C. difficile

recurrence Relative risk of recurrence at up to 4, 12, and 24 weeks after treatment Cohort A: Tx with 104 spores (n=10); placebo (n=2); single dose Cohort B: Tx with 105 spores (n=10); placebo (n=2); single dose Cohort C: Tx with 106 spores

(n=10); placebo (n=2); single dose Cohort D: Tx with 107 spores (n=10); placebo (n=2); single dose 60+ patients with primary C. difficile infection Cohort E: Tx with 108 spores (n=10); placebo (n=2); single dose Multi Dose Cohorts: Tx spores (n=10);

placebo (n=2); Dosing provided over three days Top line results expected in early 2018

SER-287 and Ulcerative Colitis

Leading the Microbiome Revolution

Inflammatory Bowel Disease (IBD)

opportunity for new mechanistic approaches Significant need for improved therapies Large US population: ~700K ulcerative colitis, ~700K Crohn's Fewer than ~1/3 of patients achieve remission with current therapies Many therapies are

immunosuppressive, limiting widespread use

Modulation of the microbiome is an

attractive therapeutic target for Ulcerative Colitis May address drivers of inflammation, barrier integrity, innate immune activation, and adaptive immune education and cell trafficking Effector molecules may include short chain fatty acids,

secondary bile acids, tryptophan metabolites, and TLR ligands Potentially synergistic effect with other UC products Steroids Thiopurines / MTX Anti-TNFs JAK Inhibitors Anti IL12/23 Microbiome Anti-Integrins S1P1 Agonists Gut Lumen Lamina Propria

Blood vessel Gut Epithelium

Selected references: Paramsothy et

al. Lancet, 2017; Moayyedi et al. Gastroenterology, 2015; Review article: Costello et al. Alimentary Pharmacology & Therapeutics, 2017. Microbiota transplantation provides clinical proof of concept Subject A, Baseline Subject A, 8-wk post FMT

SER-287 Phase 1b Ulcerative Colitis

study 58 mild-moderate UC patients failing standard of care* * Study designed to enroll 55 patients, with 15 in SER-287 treatment arms and 10 in the placebo / placebo arm Placebo pre-treatment for 6 days Placebo pre-treatment for 6 days Vancomycin

pre-treatment for 6 days Vancomycin pre-treatment for 6 days SER-287 once weekly for 8 weeks Placebo once daily for 8 weeks SER-287 once daily for 8 weeks SER-287 once weekly for 8 weeks (n=11) (n=15) (n=15) (n=17)

New data New data SER-287 Phase 1b

study endpoints Primary Objectives Safety and tolerability Change in composition of intestinal microbiome at 8 weeks Secondary Objectives Remission, endoscopic improvement, and response through measure of the total modified Mayo Score Change in

serum and fecal biomarkers Pathologic changes in mucosal biopsies (i.e., histology)

Clinical efficacy endpoints Endpoint

Protocol Definition Remission Total Modified Mayo Score <=2 and an endoscopic subscore of 0 or 1 Endoscopic Improvement Decrease in endoscopic subscore of >=1 Response Decrease of >=3 points in Total Modified Mayo Score from baseline, along

with either a decrease of >=1 point in rectal bleeding subscore or absolute rectal bleeding subscore of 0 or 1 Modified Mayo score components Mucosal Appearance by endoscopy Stool Frequency Rectal Bleeding Physician Rating of Disease Activity

Significant and dose dependent

impact on remission Placebo pretreatment / Placebo Vancomycin pretreatment / SER-287 daily Placebo pretreatment / SER-287 weekly Vancomycin pretreatment / SER-287 weekly p = 0.0237 (0/11) (6/15) (2/15) (3/17) Endoscopy readings were centrally read

by blinded readers. Data based on an intent to treat missing data counted as a failure analyses. Under observed data analysis, 1/10 (10%) and 6/15 (40%) patients in the placebo pretreatment / placebo and vancomycin pretreatment / SER-287 daily

treatment arms, respectively, achieved remission and endoscopic improvement (p=0.1794). The observed analysis includes a patient in the placebo study arm who experienced a disease flare and was treated with corticosteroids (a protocol violation)

prior to the end of treatment endoscopy

Dose dependent impact on endoscopic

improvement Placebo pretreatment / Placebo Vancomycin pretreatment / SER-287 daily Placebo pretreatment / SER-287 weekly Vancomycin pretreatment / SER-287 weekly p = 0.178 (1/11) (6/15) (5/15) (4/17) Endoscopy readings were centrally read by blinded

readers. Data based on an intent to treat missing data counted as a failure analyses. Under observed case analysis, 1/10 (10%) and 6/15 (40%) patients in the placebo pretreatment / placebo and vancomycin pretreatment / SER-287 daily treatment arms,

respectively, achieved endoscopic improvement (p=0.1794). The observed analysis includes a patient in the placebo study arm who experienced a disease flare and was treated with corticosteroids (a protocol violation) prior to the end of treatment

Pre-treatment endoscopy showing the

sigmoid colon with spontaneous bleeding and ulceration Post-treatment day 64 endoscopy Illustrative endoscopy improvement findings from patient in SER-287 daily treatment arm

Response rate is less reliable

endpoint; Not recommend by FDA as a primary endpoint for UC Placebo pretreat / Placebo Vancomycin pretreat / SER-287 daily Placebo pretreat / SER-287 weekly Vancomycin pretreat / SER-287 weekly Ulcerative Colitis: Clinical Trial Endpoints Guidance

for Industry2 (5/11) (9/15) (6/15) (4/17) Jairath V. et al., Journal of Crohn's and Colitis, 2016 August 2016 FDA draft guidance Data based on an intent to treat missing data counted as a failure analyses. Under observed case analysis, 6/10 (60%)

and 6/10 (60%) patients in the placebo pretreatment/placebo and vancomycin pretreatment/SER-287 daily treatment arms, respectively, achieved response (p=0.99). The observed analysis includes a patient in the placebo study arm who experienced a

disease flare and was treated with corticosteroids (a protocol violation) prior to the end of treatment endoscopy "We currently recommend a primary endpoint of clinical remission (responder definition based on Stool Frequency, Rectal Bleeding,

and Endoscopy scores)." High placebo response rate reported in other UC clinical studies using drugs with diverse mechanisms1

Histological healing RHI score

change 8 weeks post SER-287 administration Placebo pretreatment / Placebo Vancomycin pretreatment / SER-287 daily Placebo pretreatment / SER-287 weekly Vancomycin pretreatment / SER-287 weekly Note: Intent to treat population, missing data equal

failure Subjects with normal histology at Baseline were excluded. Seres also evaluated potential biomarkers serum CRP and fecal calprotectin and observed no statistically significant impact.

Favorable SER-287 Phase 1b safety

profile SER-287 daily arm demonstrated a similar safety profile to placebo No serious drug-related adverse events No subject discontinuations in the SER-287 daily treatment arm Reduced gastrointestinal adverse events provide an independent

assessment of efficacy with decreased disease activity SER-287 daily arm GI AEs: 2/15 (13.3%) vs. placebo arm: 5/11 (45.5%)

Analyses of post SER-287 treatment

impact on disease activity SER-287 Phase 1b patients were followed for up to 26 weeks post treatment: Of the 11 patients treated with SER-287 who achieved clinical remission, no patients experienced a disease flare in the 26 weeks following the end

Adapted from Leerink Nov. 27 2017

report: Future of IBD: Category should double by 2023 despite GED-0301 disappointment; Note that study-to-study differences limit the ability to directly compare results. Remission Rates for Induction in Active UC SER-287 FDA approved for UC In

development for UC Treatment Placebo Treatment minus placebo Favorable SER-287 efficacy relative to selected approved and development stage UC drugs

New data New data SER-287 Phase 1b

study endpoints Primary Objectives Safety and tolerability Change in composition of intestinal microbiome at 8 weeks Secondary Objectives Remission, endoscopic improvement, and response through measure of the total modified Mayo Score Change in

serum and fecal biomarkers Pathologic changes in mucosal biopsies (i.e., histology)

Robust SER-287 species engraftment;

highest in most efficacious study arm Statistically significant engraftment in vanco pre-treat / SER-287 daily arm, versus placebo pre-treat / placebo arm, beginning at day 7 and maintained throughout the dosing period Statistically significant and